FROM STREETS TO PATTERNS
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The components of a constitutional code will reflect the first four structural conditions of the tree analogy (Figure 7.3), to which we can add in connection type (junction type). 1. 2. 3. 4. 5.
Different types – e.g. street, square, transit mall. Ranked order – e.g. 20 km/h, 30 km/h, 40 km/h. Arteriality – based on transit-oriented arteriality. Access constraint – based on speed differential. Allowable junction type for given connecting road types.
Since the combined specification of arteriality and access constraint generates a ranked order of different types, this actually boils down to a minimum of just three considerations (3, 4 and 5) for a transport route constitution, although, for a street-based constitution, it is likely that some variations in type or ordering that go beyond network connection factors would also be helpful. First, we will look at individual street types expressed in constitutional terms, and then look at example constitutional structures demonstrated using a graphical device that will be referred to as a constitutional archetype.
Street type Figure 9.9 depicts a graphical code specifying necessary and allowable connections for two street types (I and IV) in a conjoint constitution. For each street type (expressed at the left-hand side) one can read along which other types may be connected to, and which type must be connected to. Figure 9.10 gives another example, this time of a dendritic constitution. Here, access constraint applies as well as arteriality, and so there will be a reduced number of allowable connections. Any street type can be graphically depicted in this way. As well as individual streets being coded in this way, we can also depict a whole street typology (constitution) in a single diagram, which will be referred to as a constitutional archetype.
Constitutional archetype An archetype is a general model that embodies the features of all individual cases. In biology, the concept of an archetype was used to try to explain the diversity and similarity between different species and body plans. Philip Steadman has applied the concept of the archetype to buildings, where any building can be expressed as a combination of variations from a basic ‘archetypal building’.6